Condensed Financial Information (Parent Company Only) - Condensed Statements of Income (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Condensed Financial Statements, Captions
Salaries and benefits	$ 96,513	$ 94,002	$ 89,833
Interest expense	18,606	20,695	30,114
Acquisition Expense	4,017	0	0
Earnings before income tax benefit	129,615	132,702	88,262
Income tax benefit	45,214	46,566	30,038
Net income	84,401	86,136	58,224
Parent Company
Condensed Financial Statements, Captions
Dividends from subsidiaries	58,900	71,400	40,000
Other interest income	42	34	92
Other income, primarily management fees from subsidiaries	12,166	12,809	10,042
Total income	71,108	84,243	50,134
Salaries and benefits	15,722	15,914	13,205
Interest expense	4,002	4,098	6,691
Acquisition Expense	4,017
Other operating expenses, net	7,512	7,546	7,150
Total expenses	31,253	27,558	27,046
Earnings before income tax benefit	39,855	56,685	23,088
Income tax benefit	(6,862)	(5,703)	(6,222)
Income before undistributed earnings of subsidiaries	46,717	62,388	29,310
Undistributed earnings of subsidiaries	37,684	23,748	28,914
Net income	$ 84,401	$ 86,136	$ 58,224